1. Field of the Invention
The present invention relates to an electrophotographic toner useful for one-component or two-component developers. In addition, the present invention relates to an image forming method and apparatus, such as copiers, printers and facsimile machines, using the toner.
2. Discussion of the Background
At the present day, various electrophotographic image forming methods are known. The methods typically include the following processes:
(1) the surface of an image bearing member (a photoreceptor) is charged (charging process); n(2) the image bearing member (photoreceptor) is exposed to light to form an electrostatic latent image thereon (latent image forming process); PA0 (3) the latent image is developed with an electrophotographic toner to form a toner image on the image bearing member (developing process); PA0 (4) the toner image is transferred onto a receiving material (transferring process); and PA0 (5) the toner image on the receiving material is fixed upon application of heat or pressure, or combination thereof (fixing process).
Thus, a document having a fixed image is provided. The toner remaining on the image bearing member even after the toner image is transferred on the receiving material is removed by a known cleaning method using a blade, brush, roller or the like.
Digital image formation and high quality image formation are the recent trends of the electrophotography. With respect to resolution of images, it is needed and investigated to form an image having high resolution of not less than 1200 dpi (dot per inch). In order to prepare images having such high resolution, a toner having a relatively small particle diameter is needed. For example, Japanese Laid-Open Patent Publications Nos. 1-112253, 2-284158, 3-181952 and 4-162048 have proposed toners having a small particle diameter and a specified particle diameter distribution.
In the toners having a small particle diameter, the adhesion force of a toner particle to other toner particles or members (photoconductor etc.) of an image forming apparatus cannot be neglected. This adhesion force influences almost all the image forming processes in an image forming apparatus, and thereby the image qualities are influenced. In particular, the adhesion force causes background development in the developing process. In addition, the adhesion force causes hollow defects and scattered toner images generated in the transferring process. Further, the adhesion force causes background fouling due to toner particles remaining on the photoreceptor after the cleaning process. When a toner having a relatively small particle diameter is used, these phenomena are furthered. Therefore, when designing a toner, it is an important subject how to control the adhesion force of the particles of the toner.
For example, in a developing process using a two component developer, at first a large amount of toner particles contact (or adhere to) the surface of a photoreceptor in a developing nip. The toner particles adhere to the photoreceptor (i.e., the toner is used for developing) or return to (i.e., are scavenged by) the carrier particles depending on the direction and quantity of Coulomb force due to the developing determined depending on the charge quantity of the toner particles. However, the charge quantity of the toner particles has a distribution, and therefore the behaviors of toner particles are different depending on their charge quantity.
In general, a developing bias is applied to a developing nip to avoid the background development of images. The toner particles, which are present in the background area of the photoreceptor in the developing nip and which are normally charged, are scavenged by the carrier particles due to large Coulomb force formed in the developing nip, and thereby occurrence of background development can be avoided. However, a small amount of toner particles having an opposite charge are present in a toner. These toner particles receive Coulomb force such that the toner particles adhere to the photoreceptor, and thereby background development occurs. In addition, toner particles having a relatively low charge quantity, the Coulomb force formed toward the carrier particles is low. Therefore the toner particles tend not to be scavenged by the carrier particles, and tend to remain on the photoconductor.
Thus, the reason of background development is considered to be that the toner particles having an opposite charge or a relatively low charge quantity included in toner particles adhere to the background area. The toner particles having such an abnormal charge are formed when a toner and a carrier are insufficiently mixed and stirred. In addition, the toner particles having such an abnormal charge increase when the toner is used for a long period of time.
In attempting to solve this problem (i.e., to reduce the toner particles having such an abnormal charge), a technique that a toner having a specified charge distribution is used is disclosed in Japanese Laid-Open Patent Publication No. 4-110861. However, when a toner having a relatively small particle diameter or a toner having a relatively low softening point which is used for low heat energy fixing is used, the background development problem cannot be solved by such a technique. Therefore, it is important to control the adhesion force of toner.
In the transferring process, toner particles on a photoreceptor are transferred onto a receiving material due to an electrostatic field formed in the transferring area. The toner transfer depends on the amount of the adhesion force of the toner and the amount of the electrostatic field. Therefore, it is important to control the adhesion force of toner. When the toner transfer is performed using a roller or a belt, the toner images on a photoreceptor are pressed toward the photoreceptor. Therefore, the toner particles of the images tend to gather on the surface of the photoreceptor, resulting in increase of adhesion force between the toner particles and the photoreceptor, and increase of adhesion force among the toner particles. Therefore, the toner tends to remain on the photoreceptor. This phenomenon tends to particularly occur in center parts of line images, resulting in occurrence of hollow defects. Accordingly, it is needed to control the adhesion force between the toner particles and the photoreceptor, and the adhesion among the toner particles. Alternatively, the pressure applied to the toner may be decreased.
The adhesion force between the toner particles and the photoreceptor is classified into an electrostatic adhesion force depending on the charge quantity of the toner particles and a non-electrostatic adhesion force. When the charge quantity of toner particles is decreased, the electrostatic adhesion force can be decreased. However, the charge quantity of toner particles is too low, a problem which occurs is that toner images cannot be transferred by the electrostatic filed formed in the transferring area.
The adhesion force has been discussed in Japanese Laid-Open Patent Publications Nos. 5-333757, 6-167825, and 6-167826. However, the non-electrostatic adhesion force is not discussed therein. In Japanese Laid-Open Patent Publication No. 8-305075, the non-electrostatic adhesion force among toner particles is discussed, however the relationship between image qualities and non-electrostatic adhesion force of toner particles to members in an image forming apparatus is not discussed.
The adhesion force among toner particles or between toner particles and members in an image forming apparatus has a distribution. When the distribution is broad (i.e., there are toner particles having too large adhesion force or too small adhesion force), image defects tend to be produced. Therefore, toner having adhesion force whose distribution is sharp is preferable. Since the adhesion force of toner particles generally depends on the particle diameter, a toner having a particle diameter whose distribution is sharp is needed to obtain a toner having an adhesion force whose distribution is sharp. In order to prepare a toner having a particle diameter whose distribution is sharp, the particle diameter of toner particles is severely controlled in the pulverization process and/or the classification process when the toner is manufactured. In attempting to prepare such a toner having a particle diameter whose distribution is sharp, Japanese Laid-Open Patent Publications Nos. 5-134455, 6-273978 and 7-333890 disclose toner manufacturing techniques. Since the classification of toner particles is severely performed in these techniques, a large amount of toner particles, whose particle diameter is outside the desired range, are disposed of, resulting in serious increase of manufacturing costs. In addition, these techniques are not preferable in the viewpoint of environmental protection.
Because of these reasons, a need exists for a toner which has adhesion force whose distribution is sharp and which is easily and efficiently manufactured.